Mechanism for increase in expression of cerebral diazepam binding inhibitor mRNA by nicotine: involvement of L-type voltage-dependent calcium channels

Increase in diazepam binding inhibitor expression by sustained morphine exposure is mediated via μ-opioid receptors in primary cultures of mouse cerebral cortical neurons

Our previous in vivo experiment demonstrates that chronic morphine treatment up-regulates diazepam binding inhibitor (DBI) transcripts in mouse cerebral cortex, although detailed mechanisms were unclear (Katsura et al. [1998] J. Neurochem. 71:2638-2641). This study sought to elucidate the precise mechanisms of DBI mRNA up-regulation by long-term treatment with morphine using primary cultures of mouse cerebral cortical neurons. A significant increase in DBI mRNA was observed after sustained exposure to 0.3 microM morphine for 2 days, and the maximal expression occurred after 2 days of exposure, whereas transient exposure to 0.3 microM morphine for 15 min, 1 hr, and 3 hr produced no changes in the expression. Continuous exposure to DAMGO also significantly increased DBI mRNA expression, which was completely abolished by a selective antagonist of mu-opioid receptors, beta-funaltrexamine (beta-FNA). The morphine-induced increase in DBI mRNA expression and its content were completely inhibited by naloxone and beta-FNA, and the inhibitory potential of naloxonazine was about half that of beta-FNA. On the other hand, kappa- and delta-opioid receptor antagonists showed no effects on the morphine-induced increase in DBI mRNA. In addition, both a calmodulin antagonist and a CaM II kinase inhibitor significantly suppressed the morphine-induced increase in DBI mRNA. These results indicate that the increase in DBI expression is induced by continuous activation of mu-opioid receptors but not of kappa- and delta-opioid receptors and is regulated by the calcium/calmodulin-related phosphorylation system.

Up-regulated l-type high voltage-gated calcium channels cause increase in diazepam binding inhibitor induced by sustained morphine exposure in mouse cerebrocortical neurons

Mechanisms of increase in diazepam binding inhibitor (DBI) mRNA expression in mouse cerebrocortical neurons after sustained morphine exposure were investigated. Increases in DBI and its mRNA expressions induced by sustained morphine (0.3 microM) exposure for 3 days were completely abolished by naloxone and nifedipine, but not by omega-agatoxin VIA and omega-conotoxin GIVA. Increase in [(3)H]diltiazem binding to the particulate fractions from the morphine-treated neurons was due to increased B(max) value with no changes in K(d) value. Western blot analysis on L-type high voltage-gated calcium channel (HVCC) subunits revealed the increased expressions of alpha1C, alpha1D, and alpha2/delta1 subunits and decreased of beta4 subunit expression, whereas expression of N- and P/Q-type HVCC subunits was not changed. These results indicate that morphine-induced increase in DBI mRNA expression is mediated via increased Ca(2+) entry through up-regulated L-type HVCCs.

Bigenomic functional regulation of all 13 cytochrome c oxidase subunit transcripts in rat neurons in vitro and in vivo

Cytochrome c oxidase is a multisubunit, bigenomically encoded inner mitochondrial membrane protein. Its enzymatic activity and amount in the brain vary with metabolic demands, but the precise regulation of all 13 subunits to form a functional holoenzyme in a 1:1 stoichiometry is not well understood. To determine if all 13 subunit transcripts were coordinately regulated by functional alteration in neurons, cultured primary neurons were depolarized by potassium chloride for 5-24 h, or tetrodotoxin inactivated for 2-6 days. In vivo studies were done on rats monocularly enucleated for 4 days to 2 weeks. Expressions of cytochrome c oxidase subunit mRNAs were measured by real-time quantitative polymerase chain reaction. Results showed that in vitro, all 13 transcripts were significantly up-regulated after 5 h of depolarizing stimulation. With tetrodotoxin blockade, however, the three mitochondrial-encoded transcripts were down-regulated earlier than the 10 nuclear ones (2 days versus 4 days). In vivo, all three mitochondrial-encoded subunit mRNAs were also down-regulated earlier than the nuclear ones in deprived visual cortex (4 days versus 1 week after monocular enucleation). Cytochrome c oxidase activity and protein levels were significantly decreased in parallel after 4 days of deprivation in vitro and 1 week in vivo. Our results are consistent with a coordinated mechanism of up-regulation of all 13 transcripts in response to functional stimulation, but an earlier and more severe down-regulation of the mitochondrial transcripts than the nuclear ones in response to functional deprivation. Thus, the mitochondrial subunits may play a more important role in regulating cytochrome c oxidase protein amount and activity in neurons. Our results also point to the need of all 13 subunits to form a functional holoenzyme.

Increased expression of L-type high voltage-gated calcium channel α1 and α2/δ subunits in mouse brain after chronic nicotine administration

We investigated the effect of chronic nicotine administration on high voltage-gated calcium channels (HVCCs) in the mouse cerebral cortex. The treatment significantly increased expression of alpha1C, alpha1D, alpha1F, and alpha2/delta1 subunits with no changes of beta4 subunit of L-type HVCCs. [(3)H]Diltiazem binding to the particulate fractions increased with increased Bmax value. These results indicate that chronic nicotine treatment up-regulates L-type HVCCs, which is due to increased expression of alpha1 and alpha2/delta1 subunits.

Functional Proteins Involved in Regulation of Intracellular Ca2+ for Drug Development: Chronic Nicotine Treatment Upregulates L-Type High Voltage-Gated Calcium Channels

Neurochemical mechanisms underlying drug dependence and withdrawal syndrome remain unclear. In this review, we discuss how chronic nicotine exposure to neurons affects expression of diazepam binding inhibitor (DBI), an endogenous anxiogenic neuropeptide supposed to be a common substance participating drug dependence, and function of L-type high voltage-gated Ca(2+) channels (HVCCs). We also discuss the functional interaction between DBI and L-type HVCCs in nicotine dependence. Both DBI levels and [(45)Ca(2+)] influx significantly increased in the brain from mice treated with nicotine for long term, which was further enhanced after abrupt cessation of nicotine and was abolished by nicotinic acetylcholine receptor (nAChR) antagonists. Similar responses of DBI expression and L-type HVCC function were observed in cerebral cortical neurons after sustained exposure to nicotine. In addition, increased DBI expression was inhibited by antagonists of nAChR and L-type HVCCs. Sustained exposure of neurons to nicotine significantly enhanced expression of alpha(1) and alpha(2)/delta(1) subunits for L-type HVCCs and caused an increase in the B(max) value of [(3)H]verapamil binding to the particulate fractions. Therefore, it is concluded that the alterations in DBI expression is mediated via increased influx of Ca(2+) through upregulated L-type HVCCs and these neurochemical changes have a close relationship with development of nicotine dependence and/or its withdrawal syndrome.

Pharmacological Basis for Management of Drug Dependence

Neurochemical mechanisms underlying development of drug dependence and withdrawal syndrome remain unclear. Several clinical features of withdrawal syndrome, such as anxiety, are considered to be common among patients with drug dependence induced by different drugs of abuse. In the present study, we investigated whether diazepam-binding inhibitor (DBI), an endogenous anxiogenic neuropeptide, participates in the anxiety associated with drug dependence and its withdrawal symptoms. When we examined brain from mice dependent on alcohol, nicotine, and morphine, we observed that the levels of DBI protein and its mRNA significantly increased. Abrupt cessation of these drugs facilitated further increases in DBI expression. In the cases of nicotine- and morphine-dependent mice, concomitant administration of specific antagonists for nicotinic acetylcholine and m-opioid receptors, respectively, abolished the increased expression. Similar patterns of DBI expression were observed in the neurons after sustained exposure to these drugs and its removal from culture medium. Sustained exposure of the neurons to abused drugs significantly increased the KCl (30 mM)-induced 45Ca2+ influx and enhanced expression of alpha1 subunits for L-type high voltage-gated Ca2+ channels (HVCCs). In addition, the increase in DBI expression was completely blocked by L-type HVCC inhibitors. Therefore, these alterations in DBI expression, mediated via increased influx of Ca2+ through upregulated L-type HVCCs, are closely related to drug dependence and/or its withdrawal syndrome and are considered to be part of a common biochemical process in drug dependence induced by different drugs of abuse.

L-type high voltage-gated calcium channels cause an increase in diazepam binding inhibitor mRNA expression after sustained exposure to ethanol in mouse cerebral cortical neurons

Mechanisms for increase in diazepam binding inhibitor (DBI) mRNA expression after sustained exposure to ethanol (EtOH) were investigated. Increases in 30 mM KCl-induced [45Ca(2+)] influx and DBI mRNA expression after EtOH (50 mM) exposure for 3 days were completely abolished by nifedipine, but not by omega-agatoxin VIA and omega-conotoxin GIVA. These results indicate that EtOH-induced increase in DBI mRNA expression is mediated via increased Ca(2+) entry through up-regulated L-type high voltage-gated calcium channels.

Up-regulation of L-type voltage-dependent calcium channels after long term exposure to nicotine in cerebral cortical neurons

Effects of long term (72-h) exposure to low concentration (0.1 mum) of nicotine on various types of voltage-dependent Ca(2+) channels (VDCCs) and neuronal nicotinic acetylcholine receptors (nnAChRs) were examined using primary cultures of mouse cerebral cortical neurons. High potassium (30 mm KCl)-stimulated (45)Ca(2+) influx into the neurons increased with increasing the duration of nicotine exposure and its concentrations. The maximal increase of the KCl-stimulated (45)Ca(2+) influx was found 24 h after the initiation of exposure and thereafter maintained up to 72 h. This enhancement of KCl-induced (45)Ca(2+) influx after 72-h exposure to 0.1 mum nicotine was completely abolished by concomitant exposure with mecamylamine, an inhibitor for nnAChRs. Only the component of the KCl-induced (45)Ca(2+) influx observed after long term exposure to nicotine, which was sensitive to nifedipine, an inhibitor of L-type VDCCs, was facilitated, while the (45)Ca(2+) influx through P/Q- and N-type VDCCs showed no changes. Moreover, enhanced immunoreactivity against antibody for the alpha(1C) subunit of L-type VDCCs was recognized, whereas no changes in immunoreactivities against antibodies for alpha(1A) and alpha(1B) subunits of other types of VDCCs were noted. In addition, a Western blot analysis showed an increase of immunoreactivities against antibodies for alpha(1D) and alpha(2)/delta(1), and expression of mRNA for L-type VDCC subunit, alpha(1F), was also enhanced, although beta(4) mRNA expression was not changed. Whole cell patch clamp analysis revealed that the increase of the amplitude of Ba(2+) currents was also recognized in the neurons exposed to nicotine, and nicardipine reduced this increased amplitude to the level of the amplitude detected in nontreated neurons with nicardipine. The up-regulation of alpha(4) and beta(2) subunits, but not the alpha(3) subunit of nnAChRs, was also noted after the nicotine exposure when examining by the Western blot analysis. Taken together, these results indicate that the long term exposure of the neurons to a low concentration of nicotine induces both increased (45)Ca(2+) influx through up-regulated L-type VDCCs and nnAChR up-regulation.

Withdrawal from nicotine facilitates diazepam binding inhibitor mRNA expression in mouse cerebral cortex

Changes in diazepam binding inhibitor (DBI) mRNA expression after withdrawal from nicotine were examined. Withdrawal from nicotine Increased DBI mRNA expression in cerebral cortices derived from nicotine-dependent mice and in the neurons continuously exposed to nicotine (0.1 microM). These results indicate that withdrawal from nicotine after its long-term exposure induces steep increase of DBI mRNA expression as reported previously in ethanol- and morphine-dependent animals.

[Functional involvement of cerebral diazepam binding inhibitor (DBI) in the establishment of drug dependence]

Mechanisms for formation of drug dependence and emergence of withdrawal syndrome are not yet fully understood despite of a huge accumulation of experimental and clinical data. Several clinical features of withdrawal syndrome are considered to be common (i.e., anxiety) among patients with drug dependence induced by different drugs of abuse. In this review, we have discussed the possibility of the functional involvement of diazepam binding inhibitor (DBI), an endogenous neuropeptide for benzodiazepine receptors with endogenously anxiogenic potential, in the development of drug dependence and emergence of its withdrawal symptom. The levels of DBI protein and its mRNA significantly increased in the brain derived from mice dependent on alcohol (ethanol), nicotine and morphine, and abrupt cessation of these drugs facilitated further increase in DBI expression. In the cases of nicotine- and morphine-dependent mice, concomitant administration of antagonists for nicotinic acetylcholine and opioid receptors, respectively, abolished the increase in DBI expression. Therefore, these alterations in DBI expression have a close relationship with formation of drug dependence and/or emergence of withdrawal syndrome and are considered to be a common biochemical process in drug dependence induced by different drugs of abuse.